The present invention relates generally to exposure devices, image-forming devices, and manufacturing methods of the exposure devices. The present invention is suitable, for example, for an exposure device and electrophotographic recording device that utilize an LED for an optical system to form multicolor images. The xe2x80x9celectrophotographic recording devicexe2x80x9d by which we mean is a recording device employing the Carlson process described in U.S. Pat. No. 2,297,691, as typified by a laser printer, and denotes a nonimpact image-forming device that provides recording by depositing a developer as a recording material on a recordable medium (e.g., printing paper, and OHP film). The electrophotographic recording device capable of forming multicolor images, which is also called a color tandem printer, typically uses a plurality of optical heads, and arranges a plurality of image-forming units each having such a head in tandem. The inventive image-forming device is applicable not only to a discrete printer, but also generally to various apparatuses having a printing function such as a photocopier, a facsimile unit, a computer system, word processor, and a combination machine thereof.
With the recent development of office automation, the use of electrophotographic recording devices for computer""s output terminals, facsimile units, photocopiers, etc. has spread steadily. Specifically, fields of color laser printers and PPC color copiers having an image-processing feature that combines microprocessors with color scanners, for example, are expected to increasingly demand multicolor printing rather than mono-color printing in the near future.
The electrophotographic recording device capable of multicolor printing typically includes a plurality of image-forming units and one fixer. Each image-forming unit and the fixer are generally aligned in line. Since multicolor images are normally formed by a combination of cyan (C), magenta (M), yellow (Y), and black (K), four image-forming units are provided in general. Each image-forming unit generally includes a photoconductive insulator (photosensitive drum), a (pre-) charger, an exposure device, and a transfer part.
The charger electrifies the photosensitive drum uniformly (e.g., at xe2x88x92600 V). The exposure device, using an optical system such as an LED, irradiates a light from its light source, and varies a potential on an irradiated area, for example, to xe2x88x9250 V or so, forming electrostatic latent images on the photosensitive drum. The LED optical system is a device in which LED chips by the number of recording pixels are placed in line to make exposure to light through an unmagnified erect image-forming optical system such as SELFOC(trademark) lens array, and a beam from an LED array is led, for instance, onto the photosensitive drum with the SELFOC(trademark) lens array.
A development device electrically deposits a developer onto the photosensitive drum using, for example, a reversal process, and visualizes a latent image into a toner image. The reversal process is a development method that forms an electric field by a development bias in areas where electric charge is eliminated by exposure to light, and deposits the developer having the same polarity as uniformly charged areas on the photosensitive drum by the electric field. The transfer part, for example, using a corona charger, transfers the toner image corresponding to the electrostatic latent image on the medium.
Each step of charging, exposure to light, development, and transfer is repeated four times for four colors with respect to four image-forming units, and thereby four-color multi-layered toner (toner multi-layers) are formed on the medium. Toner multi-layers are fixed on the medium using the fixer. To be more specific, the fixer melts and fixes the toner image by applying heat, pressure or the like, and forms a color image on the medium. The fixer for the multicolor image-forming device fixes toner multi-layers for four colors, and therefore requires higher fixing energy and thus generates more intense heat than that of a single-color image-forming device.
The post processes may include charge neutralization and cleaning on the photosensitive drum from which toner is transferred out, a collection and recycle and/or disposal of residual toner, etc. As described above, the multicolor image is expressed by a combination and superimpose of four colors.
A conventional multicolor image-forming device, however, would disadvantageously cause a thermal expansion of one exposure device under such an environment in temperature as different from other exposure devices, and results in a deviation of colors in a final image. A cause of such a color deviation lies in the exposure device nearest the fixer, and the color deviation would occur particularly in printing immediately after an idle period (i.e., suspension period). After diverse investigations, the present inventors have discovered that stored heat in the fixer causes the color deviation.
During continuous printing, four image-forming units are more or less uniformly influenced by heat generated in a whole device, and thus each exposure device thermally expands uniformly. However, the fixer has a feature that heat generated therein during a printing operation is not dissipated immediately after the suspension of continuous printing operation but stored inside for a long time. Thus, during idle time, the image-forming unit nearest to the fixer is heated by radiation and conduction of residual heat in the fixer, and other three image-forming units, as apart from the fixer, are cooled in sequence. In other words, the exposure device of the image-forming unit nearest to the fixer is put in an environment where its ambient temperature is higher than those of other three exposure devices during idle time. If the four exposure devices are thermally expanded in a nonuniform manner, areas to be exposed on the photosensitive drum does not match one another, causing a deviation of colors in a final image. The colors would be deviated greatly particularly in printing immediately after idle time.
On the other hand, in order to overcome the foregoing disadvantages, it is conceivable to cool the fixer by a high-performance cooler or thermally insulate the fixer from the exposure devices, but these would disadvantageously raise the size of the whole device and its price.
Therefore, it is an exemplified general object of the present invention to provide a novel and useful exposure device, image-forming device and manufacturing method of the exposure device, in which the above conventional disadvantages are eliminated.
Another exemplified and more specific object of the present invention is to provide an exposure device, image-forming device and manufacturing method of the exposure device that can lessen the deviation of colors to form a higher-quality image.
In order to achieve the above objects, an exposure device as one exemplified embodiment of the present invention comprises a first exposure unit that emits a plurality of dots at a first interval between the dots onto a photoreceptor material, and a second exposure unit that emits a plurality of dots at a second interval between the dots different from the first interval between the dots onto the photoreceptor material, wherein the interval between the dots in a specified area of the second exposure unit is shorter than the interval between the dots in an area of the first exposure unit corresponding to the specified area. Alternatively, an interval between the dots in a chip of the second exposure unit is shorter than a corresponding interval between the dots in a corresponding chip of the first exposure unit. Further alternatively, an interval between two adjacent chips of the second exposure unit is shorter than an interval between corresponding two adjacent chips of the first exposure unit. This exposure device may allows the second exposure unit to be placed in such a position where the interval of dots is likely to expand by temperature or like environmental factors, and may help reduce the absolute value of a deviation of exposing position by the first and second exposure units.
An image-forming device as one exemplified embodiment of the present invention comprises a photosensitive body, an exposure device that exposes the photosensitive body to light and forms a latent image, and a fixing device that fixes a toner image corresponding to the latent image onto a recordable medium, wherein the exposure device may comprise any one of the above-described embodiments. This image-forming device, which has the above exposure device, may thus manifest the same effects.
A manufacturing method of an exposure device as an exemplified embodiment of the present invention comprises the steps of manufacturing a plurality of exposure units including a plurality of light-emitting elements having a specified interval of dots, measuring a manufacturing error of an interval from a standard position in the exposure unit on all of the plurality of exposure units, classifying some of the exposure units of which the manufacturing error is over a standard value into a first group and the other of the exposure units of which the manufacturing error is below the standard value into a second group, after the step of manufacturing, and selecting as a first exposure unit at least one exposure unit from the first group and as a second exposure unit at least one exposure unit from the second group and manufacturing an exposure device including the first and second exposure units. This method can economically manufacture the above exposure device utilizing a variation of manufacturing errors of the exposure units.
Other objects and further features of the present invention will become readily apparent from the following description of the embodiments with reference to accompanying drawings.